NAME

SYNOPSIS

DESCRIPTION

The semget() system call returns the semaphore set identifier
associated with the argument key. A new set of nsems semaphores is
created if key has the value IPC_PRIVATE or if no existing semaphore
set is associated with key and IPC_CREAT is specified in semflg.
If semflg specifies both IPC_CREAT and IPC_EXCL and a semaphore set
already exists for key, then semget() fails with errno set to EEXIST.
(This is analogous to the effect of the combination O_CREAT|O_EXCL
for open(2).)
Upon creation, the least significant 9 bits of the argument semflg
define the permissions (for owner, group and others) for the semaphore
set. These bits have the same format, and the same meaning, as the
mode argument of open(2) (though the execute permissions are not
meaningful for semaphores, and write permissions mean permission to
alter semaphore values).
The values of the semaphores in a newly created set are indeterminate.
(POSIX.1-2001 is explicit on this point.) Although Linux, like many
other implementations, initializes the semaphore values to 0, a
portable application cannot rely on this: it should explicitly
initialize the semaphores to the desired values.
When creating a new semaphore set, semget() initializes the set’s
associated data structure, semid_ds (see semctl(2)), as follows:
sem_perm.cuid and sem_perm.uid are set to the effective user ID
of the calling process.
sem_perm.cgid and sem_perm.gid are set to the effective group ID
of the calling process.
The least significant 9 bits of sem_perm.mode are set to the
least significant 9 bits of semflg.
sem_nsems is set to the value of nsems.
sem_otime is set to 0.
sem_ctime is set to the current time.
The argument nsems can be 0 (a don’t care) when a semaphore set is not
being created. Otherwise nsems must be greater than 0 and less than or
equal to the maximum number of semaphores per semaphore set (SEMMSL).
If the semaphore set already exists, the permissions are verified.

RETURNVALUE

If successful, the return value will be the semaphore set identifier (a
non-negative integer), otherwise -1 is returned, with errno indicating
the error.

ERRORS

On failure errno will be set to one of the following:
EACCES A semaphore set exists for key, but the calling process does not
have permission to access the set, and does not have the
CAP_IPC_OWNER capability.
EEXIST A semaphore set exists for key and semflg specified both
IPC_CREAT and IPC_EXCL.
EINVALnsems is less than 0 or greater than the limit on the number of
semaphores per semaphore set (SEMMSL), or a semaphore set
corresponding to key already exists, and nsems is larger than
the number of semaphores in that set.
ENOENT No semaphore set exists for key and semflg did not specify
IPC_CREAT.
ENOMEM A semaphore set has to be created but the system does not have
enough memory for the new data structure.
ENOSPC A semaphore set has to be created but the system limit for the
maximum number of semaphore sets (SEMMNI), or the system wide
maximum number of semaphores (SEMMNS), would be exceeded.

CONFORMINGTO

SVr4, POSIX.1-2001.

NOTES

IPC_PRIVATE isn’t a flag field but a key_t type. If this special value
is used for key, the system call ignores everything but the least
significant 9 bits of semflg and creates a new semaphore set (on
success).
The following limits on semaphore set resources affect the semget()
call:
SEMMNI System wide maximum number of semaphore sets: policy dependent
(on Linux, this limit can be read and modified via the fourth
field of /proc/sys/kernel/sem).
SEMMSL Maximum number of semaphores per semid: implementation dependent
(on Linux, this limit can be read and modified via the first
field of /proc/sys/kernel/sem).
SEMMNS System wide maximum number of semaphores: policy dependent (on
Linux, this limit can be read and modified via the second field
of /proc/sys/kernel/sem). Values greater than SEMMSL*SEMMNI
makes it irrelevant.

BUGS

The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
clearly show its function.
The semaphores in a set are not initialized by semget(). In order to
initialize the semaphores, semctl(2) must be used to perform a SETVAL
or a SETALL operation on the semaphore set. (Where multiple peers do
not know who will be the first to initialize the set, checking for a
non-zero sem_otime in the associated data structure retrieved by a
semctl(2) IPC_STAT operation can be used to avoid races.)